Ultrasound has been used for non-destructive inspection of parts for many years. In the ultrasonic scanning of shafts or tubes, the part is rotated about its axis of symmetry while an ultrasonic transducer is moved parallel to the axis (i.e. along the shaft or tube) to complete a scan and detect flaws in the part.
More recently, ultrasonic inspection has been extended to small parts such as integrated circuits or other electronic components. Positioning and scanning of individual parts is too time-consuming for production line testing. However, the parts are often loosely arranged in trays or carriers to facilitate transfer of the parts through the production process. Scanning of the parts in trays speeds up the inspection process, but requires special apparatus to hold the parts in place while they are immersed in an acoustic coupling fluid or subjected to a flow of coupling fluid. Additional apparatus is required to dry the parts after they have been scanned. The scan itself is usual performed by moving the tray of parts in a series of steps in one direction while an ultrasonic transducer is moved in a perpendicular direction. In this way a raster scan of each part is performed on a rectangular grid. The process of stopping the transducer at the end of each scan and reversing its motion consumes a significant proportion of the scanning time.
An alternative to scanning individual devices is to scan a complete wafer. In this approach, a wafer is held stationary in a wafer chuck and an ultrasound transducer is moved in a rectangular grid scan pattern across the surface of the wafer.
Wafers may be rotated or spun during various stages of fabrication. Example processes utilizing wafer rotation include spinning coating, spin-drying and thickness measurement. Devices for holding and spinning wafers are commercially available.